Increasing energy demands in conjunction with the growing emphasis on the need of implementation of environmentally benign technologies have given significant thrust for developing cleaner technologies like dye sensitized solar cells (DSSCs), supercapacitors, Li-ion batteries, fuel cells etc. At the same time, future flexible and light weight electronic and electrical gadgets demand flexibility and weight reduction in the energy managing devices. Lack of flexibility in the present commercial DSSCs, supercapacitors, Li-ion batteries etc, gives major restrictions in integrating such systems with the future electronic and electric devices. Thus, it is highly important to have lighter, thinner and flexible energy converting and storing devices which in turn help the whole electric and electronic devices to become much cheaper and eco-friendly. Exchange of the individual key components such as current collector, electrode material as well as the electrolyte with lighter and flexible alternatives is the key point in the success of such devices. However, this transformation to flexibility and lighter qualities always accompanies with compromise in conductivity and electrochemical activity of the components due to the trade-off between them.
A single material possessing both high electrochemical activity and high flexibility will be promising in this context as it can play the role of both current collector and an electrode material which results in flexible, lighter, thinner, and cheaper energy devices. This approach is very challenging in the present situation due to the lack of materials which possess required conductivity, flexibility and electrochemical activity concomitantly. Among the various materials, flexible metal foils and metal coated flexible substrates possess high conductivity, and, thus, are being used as the current collectors in most of the electrochemical devices. However, issues related to electrochemical activity, cost, corrosion and density make them less viable candidates for such flexible applications. Various carbon morphologies like carbon nanotubes and grapheme are looking very promising due to their low cost and high conductivity. However, large area electrode production from highly graphitized CNT's and graphene is still challenging due to the difficulties associated with processing in solution phase. On the other hand, conducting polymers are promising in this context due to their easy processability, conductivity etc. compared to the carbon analogues.
PEDOT is a versatile conducting polymer among its counterparts owing to its very high theoretical conductivity (>500 S/cm), chemical and physical stability, largeoperable potential window etc. Thus, PEDOT is being used in various photovoltaic cells, Li-ion batteries and supercapacitors. One of the promising applications of PEDOT is in dye sensitized solar cells (DSSCs) to replace costly Pt coated FTO counter electrode. At the same time, due to its high conductivity, extensive researches are also going on to use it as a potential electrode material in supercapacitors.
For flexible counter electrode as well as supercapacitor applications, achieving low sheet resistance for PEDOT on a flexible substrate is necessary. Among the various methods available for the preparation of the PEDOT electrodes, vapour phase, refer Kim, J et al in “The preparation and characteristics of conductive poly(3,4-ethylenedioxythiophene) thin film by vapor-phase polymerization” in Synth. Met. 2003, 139, 485-489 reports preparation method of conductive PEDOT film at nano-level thickness on plastic roll film substrates by vapor-phase polymerization. The conductive thin films of ferric chloride doped poly (3,4-ethylenedioxythiophene) were obtained by in situ vapor-phase polymerization method under ambient conditions. The conductivity is also affected by depositing temperature, and strongly depended on the film thickness. It shows the high conductivity to 100 S/cm at 20-100 nm range thickness and up to 102 S/cm at above. The major drawback of said process is that the conductivity heavily depends upon film thickness and higher conductivity (100 S/cm) can be achieved only at very low thickness (20-100 nm range). The film possessing low thickness does not have significant mechanical integrity and hence cannot be employed for the fabrication of energy storage and/or conversion devices.
Article titled “Preparation and characterization of conductive paper via in situ polymerization of 3,4-ethylenedioxythiophene” by Y Chen et al. published in BioResources, 2011, 6(3), pp 3410-3423 reports conductive paper prepared via in situ chemical oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) in pulp suspension by using iron(III) p-toluenesulfonate (Fe(OTs)3) as both an oxidant and a dopant source. The deposition of poly (3,4-ethylenedioxythiophene) (PEDOT) on the pulp fiber surface was verified and characterized by ATR-FTIR and SEM analyses. The conductivity of the resultant conducting films described in said paper is less due to higher volume fraction of pulp.
Article titled “Fabrication of conductive paper coated with PEDOT: Preparation and characterization” by H Kawashima et al. published in Journal of Coatings Technology and Research, 2012, 9 (4), pp 467-474 reports a conductive paper coated with PEDOT by direct polymerization onto a paper sheet. The conductive paper exhibited the electrical conductivity of 1.8 S/cm. A conductive paper was fabricated by the EDOT monomer painting/simultaneous polymerization method for the formation of PEDOT/cellulose composite. The electrical conductivity of the PEDOT-coated paper was estimated to be 1.8 S/cm. The drawback of said process is low mechanical integrity and low conductivity of the resultant PEDOT/cellulose composite which pose limitations during the fabrication of energy storage and/or conversion devices.
A simple and common method compared to the aforementioned methods is direct coating of the chemically synthesized PEDOT on a flexible substrate via various techniques like spin coating, brush coating or bar coating. Main drawback of this method is the low processability and low conductivity of the chemically synthesized PEDOT owing to the fast polymerization rate which leads to disordered and short polymer chains with shorter π conjugation.
Article titled “Fast conductance switching in single-crystal organic nanoneedles prepared from an interfacial polymerization-crystallization of 3,4-ethylenedioxythiophene” by K Su et al. published in Advanced Material, 2007; 19(5), pp 669-672 reports the synthesis of single crystals of poly(3,4-ethylenedioxythiophene) (PEDOT) as nanoneedles, which projected fast, field-induced conductance switching by interfacial polymerization from the 3,4-ethylenedioxythiophene (EDOT).
Article titled “Single-Crystal Poly(3,4-ethylenedioxythiophene) Nanowires with Ultrahigh Conductivity” by B Cho e al. published in Nano Lett., 2014, 14 (6), pp 3321-3327 reports single-crystal poly(3,4-ethylenedioxythiopene) (PEDOT) nanowires with ultrahigh conductivity using liquid-bridge-mediated nanotransfer printing with vapor phase polymerization. The single-crystal PEDOT nanowires are formed from 3,4-ethylenedioxythiophene (EDOT) monomers that are self-assembled and crystallized during vapor phase polymerization process within nanoscale channels of a mold having FeCl3 catalysts. The conductivity of the single-crystal PEDOT nanowires is an average of 7619 S/cm with the highest up to 8797 S/cm which remarkably exceeds literature values of PEDOT nanostructures/thin films.
Article titled “Improvement of Electrical Conductivity of Poly(3,4-ethylenedioxythiophene) (PEDOT) Thin Film” by SH Yu et al. published in Molecular Crystals and Liquid Crystals, 2013, 580 (1), pp 76-82 reports effect of doping level and co-dopant on the electrical conductivity of in-situ polymerized poly(3,4-ethylenedioxythiophene) (PEDOT) thin film. PEDOT thin film was fabricated by in-situ polymerization of 3,4-ethylenedioxythiophene (EDOT) as a monomer and iron (III) p-toluenesulfonate (FTS) as the oxidant and the dopant source. The PEDOT films with very smooth surface were successfully fabricated on glass substrates by in-situ polymerization. The prepared PEDOT films showed the conductivity ranging from 700 to 1,000 S/cm.
However, practical issues to produce larger area films as well as the inability to attain comparable coating with a low sheet resistance make PEDOT a less viable choice for conceiving current collector-free electrodes.
As one of the excellent solutions to overcome this issue is the use of a retardant, normally a Lewis base, which slows down the polymerization rate. However, harmful nature of the retardantsandtheir inability to make a significant reduction in the sheet resistance makes the process less viable and attractive.
Even though interfacial polymerization is common for polyaniline, polypyrrole and polythiophene, few reports on the interfacial polymerization are available for PEDOT. Most of the reports utilize surfactants; however, surfactant always has a negative impact on the conductivity of the formed PEDOT. Yang et al (Su, K.; Nuraje, N.; Zhang, L.; Chu, I. W.; Peetz, R. M.; Matsui, H.; Yang, N. L. Adv. Mater. 2007, 19, 669-672) prepared a semi conducting PEDOT nano-needles in solution at water/dichloromethane interfaces. However, lengthy reaction time (3 days) and poor yield are the main limitations of the solution method.
Therefore, there is need to provide an efficient and cost-effective process for preparation of highly conducting PEDOT electrodes with low sheet resistance on a flexible substrate and will overcome drawbacks of prior art. Accordingly, present inventors developed a process for preparation of highly conducting PEDOT cellulose paper with low sheet resistance and high conductivity.